The present disclosure is related to a calibration fixture for use in calibrating a machine vision vehicle wheel alignment measurement system, and more particularly, to a light weight calibration fixture configured to receive optical targets in precision co-axial mountings at opposite ends of a rotating transverse bar, supported concentrically by bearings within an outer structural tube for isolation from external forces.
Machine vision vehicle wheel alignment measurement systems, and particularly those which utilize two or more imaging sensors or cameras, require periodic calibration to establish the relationship between the observed fields of view of each imaging sensor or camera, as well as the spatial relationships between various components such as the vehicle supporting runway and the observing imaging sensors or cameras. Conventional calibration procedures utilize a calibration fixture for supporting a pair of optical targets within the observed fields of view of the various imaging sensors or cameras undergoing calibration. The conventional calibration fixture is configured to support each optical target at an opposite longitudinal end of a rotating transverse bar, which in turn is disposed at a fixed and known elevation above the surface on which the calibration fixture is disposed. By affixing the optical targets at opposite ends of the rotating transverse bar, the resulting physical connection between the optical targets ensures that the optical targets can be rotated in unison about the longitudinal axis of the bar. Rotating each optical target through a common arc ensures that imaging sensors or cameras which observe one optical target will observe the same rotational movement as imaging sensors or cameras which observe the second optical target. By acquiring multiple images of the optical targets at various rotational positions, and at various locations within the observed fields of view, mathematical transforms can be calculated which calibrate the various imaging sensors or cameras, and which define the observed spatial environment in a common reference coordinate system.
Conventional calibration fixtures are generally of a heavy and rigid construction, in order to maintain a high degree of precision. As can be seen in FIG. 1, they typically consist of a pair of vertical support legs which are laterally spaced apart. One or more fixed cross-members extend between the vertical support legs, and maintain them in a fixed relationship. The rotating transverse bar is supported by, and extends between, the vertical support members. Mounting adapters for receiving the optical targets (or other types of sensors) are disposed at opposite ends of the rotating transverse bar, axially outward from the vertical support legs. An adjustable locking pin may be used to secure the transverse bar against rotation, and similar target locking knobs may be used to secure the individual optical targets within their mounting adapters. Other, more complex and cumbersome designs, such as shown in U.S. Pat. No. 7,089,776 B2 to Dale, Jr. incorporate multiple rotating transverse bars, as well as longitudinal and diagonal coupling members to establish an entire calibration framework for placement on a vehicle support structure.
During use, the calibration fixture, with the optical targets mounted there on, is positioned at various locations within the operative fields of view of the imaging sensors or cameras, such as spanning the space between a pair of adjacent vehicle support runways. Positioning of the calibration fixture requires the operator to physically lift or move the calibration fixture. If the calibration fixture is improperly lifted, such as by grabbing the rotating transverse bar, the rotating transverse bar can be distorted or damaged, and results of the calibration process can be affected. Similarly, if an operator contacts or rests against the rotating transverse bar during a calibration procedure, the resulting calibration values may not be accurate due to minute deflections or distortions in the rotating transverse bar.
Accordingly, it would be advantageous to provide a calibration fixture for use during calibration procedures of a machine vision vehicle wheel alignment measurement system, which is of a light weight construction, but which is capable of maintaining a supported pair of optical targets in accurate axial alignment. It would be further advantageous to provide a calibration fixture in which the rotating transverse bar is protected against external influences which may result in deflection or distortion of the longitudinal axis about which the bar rotates during a calibration procedure.